How do I animate spinning arm with spinning end
Set the origin of the arm at the point where you want it to rotate (note that I added a cylinder object as axis, that will be used later).



Select the blade, then shift select the arm, and press CtrlP to parent the blade to the arm.
This will make it move and  rotate with the arm.



Select the blade only and add a Copy Rotation constraint to it. On the Target object I selected the cylinder on which the arm spins, but it could be the main object as well or an emtpy.



The blade now follows the movement of the arm but the rotation on the Z axis determined by the cylinder object.